In oil and gas well completion operations, frac plugs and bridge plugs are necessary for zonal isolation and multi-zone hydraulic fracturing processes. The advantages of frac and bridge plugs made from composite materials is well established since these products significantly reduce drill-out (removal) time compared to metallic frac and bridge plugs. However, as drilling for oil and gas extends deeper, the composite frac and bridge plugs are expected to sustain higher pressures and operating temperatures. Composites are also being used more frequently in other downhole tools and product applications such as composite pipe and tubing because of better corrosion resistance compared to metals.
A frac plug or bridge plug made of composite materials typically has a central composite tubular mandrel with integral composite sleeves at both ends to restrain the functional elements of the plug (slips, cones, gland) so that the assembly both grips the well casing and seals the plug to the well casing. Often, composite sleeves are both bonded and mechanically pinned to the frac or bridge plug mandrel in order to handle operational forces. For high temperature and pressure applications of 350 degrees F. and 10,000 psi (or higher), it has been found that sleeves that are adhesive bonded to central mandrels are sometime not strong enough for the operating range. For this reason, current high performance composite frac and bridge plug products typically have multiple mechanical shear pins along with adhesive to affix the sleeve to the mandrel in order to handle set and fracking pressure loads. These mechanical pins are not only costly to drill and assemble but also provide potential leak sites where the pins are installed. Furthermore, in certain frac and bridge plug products, there is insufficient wall thickness for the tubular mandrel to adequately pin the sleeve to the mandrel.
Adhesive bonding is normally considered an effective way to attach two composite components together, in this case one or more sleeves and a mandrel of a composite frac or bridge plug. However, typical epoxy based adhesives lose strength at elevated temperatures. Frac and bridge plug operating temperatures in the 350 F range become problematic for attaching the sleeves to the mandrel on downhole plugs with only epoxy adhesive. The epoxy adhesive at 350 F for downhole applications has roughly only 40% of the lap shear strength normally achieved at room temperature. It is not always possible to increase the lap shear surface area between the sleeve and the mandrel of a frac or bridge plug due to other design constraints. For this reason, typical frac and bridge plug products often have several shear pins penetrating through the sleeve but not completely through the mandrel. The shear pins are typically ⅜ inch to ½ inch diameter and can be made from composite materials, again to facilitate drilling out the plug after use. The shear pins provide enough shear strength to offset the loss of strength for the adhesive bond at elevated temperature. The shear pins are usually bonded into the assembly when installed.
Other downhole tools and products have a similar engineering problem requiring the connection of a composite or metal sleeve, coupling or end fitting to a composite tubular product. For example, lengths of composite downhole tubing often require a sleeve connection at the ends. The technical challenge to make a reliable and strong connection between the tubing and the end fitting is similar to that of the composite frac and bridge plug.